The problems associated with switching large electrical currents are well known, and tremendous effort has been expended in attempting to overcome those problems. Among the major problems is the difficulty in switching large electrical currents to an electrical load in a sufficiently short time to achieve acceptable voltage and current rise times. Such requirements are common in supplying electrical energy to railguns, flashlamps, and other high power devices.
Present-day sources of extremely high electrical current (on the order of several megamps) and energy (on the order of tens of megajoules) cannot provide the rise times necessary to directly operate devices such as railguns, high power lasers, and other high energy-consuming devices. A switch must be provided that allows the high current source to first transfer energy to an inductive store, for example, and then transfer the current to the load circuit. The switch is required to meet two conflicting requirements. It must be capable of conducting large electrical currents for relatively long periods of time and then opening in a relatively short time interval.
One known device for transferring an electrical current to a load is an electrical circuit breaker. This device includes contacts which, when closed, short circuit the electrical load and conduct the electrical current. When the circuit breaker is operated, the contacts separate and the electrical current is diverted into the load. However, the operating time for even the fastest circuit breakers is on the order of several milliseconds, and the desired switching times are typically in the tens of microseconds.
A second device which has been used to interrupt large electrical currents and transfer them to an electrical load is an explosively activated switch. Such a device will typically include a current-carrying, aluminum tube having an explosive primer cord inside the tube and extending along its longitudinal axis. Prior to activation of the switch, the aluminum tube conducts the large electrical current and, if properly designed, will do so for the necessary length of time. Steel cutters surround the exterior of the tube. When the switch is to be activated, the primer cord is ignited and the resulting explosion drives the tube against the steel cutters and the tube is ruptured in many locations along its length. The electrical continuity between the ends of the aluminum tube is thus interrupted. Explosive devices have been reported as having opening times of approximately 50 microseconds. However, they are relatively complex and expensive devices which are good for one operation only.
Staged switches have been used wherein a current-carrying device is used in parallel with a fast opening device. The fast opening device, which may be a fuse for example, is connected in parallel with a mechanical circuit breaker or explosively activated switch and is normally designed to achieve desired compression of the electrical pulse. The operation of the circuit breaker or the activation of the explosive switch causes the electrical current to be diverted to the fast-opening fuse. For very large electrical currents, the faster operating explosive device is preferred to the relatively slow operating mechanical circuit breaker.